and the Mode of its Communication. 119 



mercury, for instance be placed in the focus of the 

 mirror A, and that a thermometer at the temperature 

 of freezing water be placed in the focus of the mirror B ; 

 what might be expected to be the result of this experi- 

 ment ? That the thermometer would fall, in conse- 

 quence of its being cooled by the accumulation of frigo- 

 rific rays proceeding from this very cold body. 



Now this is what actually happened in the celebrated 

 experiment of my ingenious friend, Professor Pictet, of 

 Geneva. 



Several attempts have been made to explain the result 

 of that experiment, on the supposition that caloric has 

 a real or material existence, and that radiant heat is that 

 substance, emitted and sent off in right lines in all direc- 

 tions from the surfaces of hot bodies. But none of 

 these explanations appear to me to be satisfactory. One 

 of the most plausible of them is that which is founded 

 on a supposition that caloric is emitted continually, 

 under the form of radiant heat, by all bodies, at all 

 temperatures, but in greater abundance by hot bodies 

 than by such as are colder; and that a body, at the 

 same time that it sends off radiant caloric in all direc- 

 tions to the bodies by which it is surrounded, receives 

 it in return, in greater or less quantities, from all those 

 bodies ; that in all cases where a body, in any given 

 time, receives more radiant caloric than it gives off, an 

 accumulation of caloric in the body takes place, in 

 consequence of which accumulation it becomes hotter, 

 but when it gives off more caloric in any given time 

 than it receives, its quantity of caloric is gradually di- 

 minished and it becomes colder ; and that a constant 

 temperature results from the quantities of caloric emit- 

 ted and received continually being equal. But besides 



